Calculating TCO for Energy

Realistically estimate electric use and cost to compare servers

Hard vs. Soft Numbers

IT load reduction is a hard number savings. Reductions to distribution losses are also hard dollar savings. When a server is unplugged, the watts it consumed are saved, and because the watts aren’t distributed, the losses experienced are saved too.

Normally, only some of mechanical load is variable, meaning that calculating the cooling cost that corresponds with server load is a very soft number. It varies widely by data center, and can be from 0 to 100 percent. If the variable portion of mechanical facility wattage is unknown, estimating it at 25 percent is reasonable.

You can, however, estimate the kW for distribution and mechanical based on the IT load. Typically 15 to 17 percent of IT load is distribution, while 100 percent of IT load is mechanical kW. Bear in mind, these are reasonable averages based on experience. They’re representative of medium to large data centers, greater than 20,000 square feet of raised floor.

The power usage effectiveness (PUE) metric may be available for the data center. This is a ratio where 1.0 of the value is the IT load and the remainder is the distribution losses and mechanical load. If we have a data-center PUE, we know the average distribution kW, so the remainder is mechanical kW. For example, if the data center PUE is 2.10, and we assume 16 percent is distribution loss, then: 2.10 – 1.00 – 0.16 = the mechanical load as a percentage of IT load, in this case 94 percent.

Tying it Together

To determine the total yearly electrical operating expense of a server:

IT load cost per year x distribution kW x mechanical kW

To determine the yearly savings if the server were decommissioned: IT load cost per year x distribution kW x (mechanical kW x variable portion).

Using our examples of IT load yearly cost of $333 in a data center with PUE of 2.10, we find:

Distribution is 16 percent of $333, or $53

Mechanical is 94 percent of $333, or $313

Variable mechanical is 25 percent of $313, or $78

Total yearly expense will be $333 plus $53 plus $313, for a total of $699. And the potential savings will be $333 plus $53 plus $78 for a total of $465.

Calculating the cost of server electric requires typical power, electric price and distribution and mechanical percentages for the data center. Calculating the potential savings requires the same, and also requires the percentage of the mechanical load that is variable for the data center.

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